The present invention pertains to rechargeable electrochemical cells and more particularly to an improved seal and method of sealing for retaining electrolyte within the casing of a rechargeable electrochemical cell.
Many rechargeable electrochemical cells, such as nickel-cadmium and nickel-metal hydride cells are made of flexible electrode plates loaded with an electrochemically active material. The electrodes are separated by a thin nonconductive separator and the assembly is spirally wound into a cylindrical configuration and inserted into a can or other container. Electrolyte is introduced into the can and is retained within and surrounding the electrodes and separator. The cell also includes a cover that cooperates with the container to provide a sealed environment within the cell in which the various electrochemical reactions occur for storage and release of electrical energy. Due to the nature of the processes involved, the container often experiences considerable pressures, in some cells reaching several hundred pounds per square inch. It is necessary to incorporate an insulating seal material between the cover and the container wall to help seal against leakage of electrolyte from the cell at the interface between the cover and the container.
It is important that the seal associated with the above cells maintain an effective sealing function throughout the life of the cell. If the sealing function is not maintained, a number of undesirable conditions adverse to effective cell operation may result. First, evaporation of the electrolyte may result in reduced performance or failure of the cell. Second, leaking electrolyte (typically a corrosive agent) may contaminate and damage components exterior to the cell.
A variety of seal designs have been employed in past attempts at resolving the problem of leaking electrolyte in these cells. Many designs incorporate a deformable seal material that is captured and compressed between a rigid disk-like cover and the cell container walls. Maintenance of the sealing function relies on the ability to retain residual compressive stresses within the deformable yet resilient seal material. Many different combinations of seal material and geometry have been used to accomplish this. For the most part, residual compressive stresses are established by highly compressing the container wall onto or around the cover plate, compressing the seal material between. If the metal container walls are compressed past their elastic limit, they will be permanently deformed. When the external forces arc withdrawn, residual stresses in the container walls maintain compression of the seal material between the walls and the cover plate. Examples of these processes are provided in U.S. Pat. Nos. 4,523,376 and 5,080,984 to Thibault et al. However, a combination of cold-flow of the seal material and gradual relaxation of the container walls often results in loss of compression and eventual leakage. Leakage of electrolyte is still a problem with most cells.
Loss of compression is addressed in part by proper selection of seal material. Seal materials for these types of cells preferably meet at least two criteria. First, the material should be capable of sustaining high compressive stress in order to maintain an effective seal. Second, the material should also be chemically inert and unaffected when surrounded by the electrolyte contained in the cell. Some amorphous polymers such as polysulfone perform better in both regards than do nylons and other crystalline polymers used in seals. Unlike crystalline polymers, polysulfone exhibits little cold flow under the stresses experienced in seal elements. Because they flow or relax very little under compression, amorphous polymers such as polysulfone are capable of providing more effective seals. The properties of these seal materials are discussed in more detail in the above patents to Thibault et al.
In a typical seal, a deformable sealing element is fabricated in a generally toroidal shape. The sealing element has an inner surface for receiving the perimeter of a circular cover plate. These two elements are brought together within the opening of the cell container. Generally, a shoulder or shelf is provided to position the seal and the end of the container wall is bent over to capture the sealing element within. The container wall is then compressed radially toward the cover plate perimeter. The portion of the sealing element in which significant compression is established is a relatively small region adjacent the perimeter of the cover plate. This relatively small region of compression is susceptible to cold-flow and gradual loss of compression. It also presents a relatively short barrier to leaking electrolyte. One method to increase this barrier is to bend the wall end over an extended portion of the seal element, crushing this portion against the outside surface of the cover plate. However, this approach is problematic. First, in bending the wall over and toward the cover plate, contact between the wall and cover plate is risked. Because the container wall and cover plate are oppositely charged when the cell is functioning, it is critical that such contact be prevented to avert shorting. The very small dimensions involved in these components and the difficulty of holding close tolerances in a commercial production environment increase this problem. Secondly, not all seal materials can be so crushed and deformed as described. Seal elements formed of crystalline polymers such as nylon can be deformed in the manner required to form such seals. However, polysulfone and similar materials that are more desirable otherwise as a seal material cannot be used in this manner due to their higher strength and hence resistance to plastic forming. If the more desired polysulfone is to be used as a seal material, another seal design must be used. What is needed is a seal design that provides an increased barrier to electrolyte leakage past the seal element and takes advantage of the greater performance of polysulfone as a seal material.